1. Field of the Invention
The present invention relates to a method of making a capacitor, in particular a solid electrolyte capacitor. The present invention also relates to a method of processing a capacitor element incorporated in such a solid electrolyte capacitor.
2. Description of the Related Art
FIG. 14 shows a known solid electrolyte capacitor (see e.g. U.S. Pat. No. 5,693,104) The solid electrolyte capacitor shown in FIG. 14 comprises a capacitor element 90 enclosed in a resin package 91. The resin package 91 also encloses part of external connection leads 92, 93. The remaining parts of the leads 92, 93 extend out of the resin package 91. One lead 92 (cathode lead) is connected to a cathode electrode 90a formed on the capacitor element 90, whereas the other lead 93 (anode lead) is connected to an anode wire. 90b extending out of the capacitor element 90. The anode lead 93 may be connected to the anode wire 90b by means of resistance welding or thermocompression bonding for example.
The capacitor element 90 is provided by forming a dielectric layer and a solid electrolyte layer in pores of a sintered porous mass, and then forming the cathode electrode 90a. The anode wire 90b is partially enclosed in the sintered porous mass. The anode wire 90b may be buried in the sintered porous mass before forming the solid electrolyte layer.
By burying the anode wire 90b in the sintered porous mass before the solid electrolyte layer is formed, however, the surface of the anode wire 90b are oxidized due to the heating treatment included in the forming process of the solid electrolyte layer. Further, the sintered porous mass is formed by sintering a compressed metal powder. If the anode wire 90b is buried before this sintering process, it becomes oxidized during that process. In any case, to connect the anode wire 90b and the anode lead 93, an oxidized film exists therebetween. Consequently, the oxidized film inhibits proper bonding (performed by compatibility, atomic diffusion or alloying) between the metal in the anode wire 90b and the metal in the anode lead 93 even if energy is applied to the connecting portion. The oxidized film may remain, for all the applied energy, at the boundary surface (interfacial alloyed layer) and weaken the connection between the anode wire 90b and the anode lead 93.
In order to eliminate the above problem, it has been proposed to remove the oxidized film on the anode wire by sandblasting or partially remove the oxidized film by making a cut in the anode wire. However, it becomes more difficult to perform these mechanical treatments as the downsizing of the capacitor element proceeds. Further, these mechanical treatments give a large load on the anode wire as well as the portion in the sintered mass where the anode wire is buried, which may lead to breakage of the anode wire or ill-conduction between the anode wire and the sintered mass (the main part of the element), thereby deteriorating electric characteristics. In particular, the anode wire becomes thinner in accordance with the downsizing of the capacitor element, whereby the ill conduction or the breakage of the anode wire is more likely to occur.